This invention relates to a process for producing unsymmetrical polyurea-urethane coated granular fertilizer particles and the coated granular fertilizer particles produced by this process. The polyurea-urethane coatings comprise a) a polyisocyanate, and b) an alkanolamine containing at least one primary or secondary amine group and at least one hydroxyl group.
Commercial particulate fertilizers are produced and marketed in several different particle types, i.e., granular, pelletized, dusts, pilled, and prilled fertilizers. Also, they may be formed of inorganic substances, organic substances, or combinations thereof. The improvements of the present invention can be applied to any of these types of particulate fertilizers.
In order to be effective in promoting plant growth, fertilizers must contain some amount of water soluble plant nutrients. These are typically in the form of water soluble compounds of nitrogen, phosphorus and potassium, alone or in combination, and often in conjunction with other elements such as, for example, calcium, boron, magnesium, zinc, chlorine, etc. Such particulate fertilizers can be made of a single component, e.g., urea, ammonium nitrate, potassium chloride, etc., or of multiple components often mixed with inert water soluble or water insoluble materials as in common fertilizers designated as 6-6-6, 4-64, 10-10-10, 20-20-5, 14-16-0, 5-20-20, and the like. In addition, specialized fertilizers may contain optional additives such as herbicides, insecticides, trace elements, iron salts, sulfur, etc. The improvements of the present invention can be applied to any of these fertilizers.
Historically, particulate fertilizers possessed a number of known defects, the most notable being the too rapid release of soluble plant food, causing phytotoxicity and the rapid depletion of the plant nutrients by leaching. Other problems included tendencies to cake and form dust. These problems are well documented in previous patents that professed solutions to one or more of the known defects, including U.S. Pat. Nos. 3,475,154, 3,259,482, 3,264,088, 3,264,089, 4,711,659, 4,772,490 and Japanese Patent 52-38361. The present invention provides additional improvements in the construction of particulate fertilizers which render them highly attrition resistant and further extended release properties.
A number of slow release coatings for particular fertilizers have previously been proposed. The process of sulfur coating urea particles is described in U.S. Pat. No. 3,342,577 and was developed in the late 1960's by the Tennessee Valley Authority (TVA) as an economical system for reducing the rate of dissolution when urea particles are applied to the soil as fertilizer. This process requires high levels of sulfur, which reduces the nutrient analysis of the fertilizer particles, and even then, imperfections remain in the coating, making it necessary to apply a sealant coating, which is composed of a mixture of 30% polyethylene resin in a 70% bright stock mineral oil.
Coatings in which preformed polymer resins are applied from solvents have been described in, for example, U.S. Pat. Nos. 3,475,154 and 3,264,089. The use of solvents creates a vapor hazard as the products are dried and the solvent evaporation step can result in pinhole imperfections in the coatings when applied.
U.S. Pat. No. 4,369,055 tried to facilitate degradation of coating materials while maintaining the function for controlling the dissolving-out rate by dispersing inorganic powders such as sulfur and talc in a low molecular weight olefin polymer. However, the disclosed coating materials are difficult to apply in uniform layers because the polymer must be maintained in the molten state.
Polyurethane coatings as disclosed in U.S. Pat. Nos. 4,711,659 and 4,969,947 require that the substrate contains a minimum quantity of reactive --NH.sub.2 groups. Thus, these are not applicable to all fertilizer compositions for which slow release properties may be desirable.
Coating of fertilizer compositions with a biodegradable polymer was described in U.S. Pat. No. 5,176,734 and 5,206,341 and Japanese Patent Application No. 146492/1991. These references disclose coating the fertilizer composition with a biodegradable coating material in a single layer. The single layer coating has difficulties controlling the dissolving-out rate of fertilizer nutrients while maintaining biodegradability simultaneously.
Japanese Patent Application No. 97561/1993 discloses a three layer coating which was prepared by using one type of biodegradable film and a water soluble resin. This coating also has difficulty in controlling both the dissolving-out rate and the biodegradability at the same time. The thickness of the coating material is disclosed as being from 500 to 2,000 .mu.m. Such a coating material requires increased cost, thus making it questionable for commercial use.
Canadian Patent Application No. 2,135,788 relates to coating fertilizer compositions with at least two types of coating materials wherein the two coating materials have different dissolving-out rates and moisture permeability in a multi-layer structure.
U.S. Pat. No. 5,538,531 also describes controlled release fertilizers and a method for their production. These controlled release fertilizers have a central mass of particulate fertilizer which contains at least one water soluble plant nutrient surrounded by a plurality of coatings. The inner coating comprises the reaction product of (A) an aromatic polyisocyanate or derivatives thereof which contain about 1.5 to 3 NCO groups per molecule and an NCO group content of 10 to 50% by weight, and (B) a polyol having from 2 to 6 hydroxyl moieties and at least one alkyl moiety containing from about 10 to 22 carbon atoms. An outer coating is also necessary. The outer coating consists essentially of an organic wax having a drop melting point of between 50 and 120.degree. C.
U.S. Pat. No. 5,599,374 relates to a process for producing sulfur-coated slow release fertilizers which have improved impact and abrasion resistance properties. This process applies liquid monomers sequentially onto the surface of hot sulfur-coated urea granules, and copolymerizes these to form a firm, tack-free, water insoluble polymer coating sealant. Suitable liquid monomers are diisocyanates such as diphenylmethane diisocyanate, and a polyol mixture of diethylene glycol (DEG) and triethanolamine (TEOA). TEOA serves both as a reactive polyol and as a catalyst. This patent attempts to overcome the deficiencies of using sulfur alone to achieve slow release properties. The polyurethane serves to coat and cover the areas of the fertilizer particle not covered by the sulfur, and thus, provide improved time release properties.
U.S. Pat. No. 5,704,962 describes compositions for treating granular fertilizers to reduce dust and to reduce caking of the fertilizers during storage. These compositions comprise fatty monoamines, specifically fatty secondary dialkylamines or mixtures thereof with fatty primary amines.
Advantages of the present invention include the fact that improved overall fertilizer release properties result at a given percentage organic encapsulant. Furthermore, the present invention does not require an isocyanate-reactive central mass so that any sequence of application of the isocyanate and the isocyanate-reactive components can be used. The present invention also allows to mix the isocyanate and the isocyanate-reactive components prior to applying them onto the fertilizer surface. While not wishing to be bound by any particular theory, it is believed that due to the difference in reactivity of the amine and the hydroxyl co-reactants, the partially cured resin does not absorb as readily into the fertilizer particles but remains on the surface to provide a more uniform coating, resulting in better release properties.